When checking usable exit descriptors, always check exit policies

We need to check exit policies whenever we're counting the usable exit descriptors. At the moment, we only check exit policies for exits in ExitNodes.

    SMARTLIST_FOREACH_BEGIN(myexits_unflagged, const node_t *, node) {
      if (node_has_preferred_descriptor(node, 0) &&
          node_exit_policy_rejects_all(node)) {
        SMARTLIST_DEL_CURRENT(myexits_unflagged, node);
        /* this node is not actually an exit */
        np--;
        /* this node is unusable as an exit */
        nu--;
      }
    } SMARTLIST_FOREACH_END(node);